During drilling operations in connection with petroleum recovery, significant amounts of oil-based drilling mud are used. The drilling mud flows to surface entraining cuttings from the drilling operation in the borehole. A significant proportion of the drilling mud is immediately separated from the drill cuttings, whilst the drill cuttings with the remaining proportion of oil-based drilling mud is treated separately.
Relatively stringent statutory requirements prevent the drill cuttings from being discharged into the surroundings. It is known to reintroduce drill cuttings in a slurrified state into a borehole, but a significant proportion of the drill cuttings are shipped to treatment facilities for such cuttings.
According to prior art, the drill cuttings are cleaned further via centrifuging, washing by means of chemicals, or via thermal treatment. During thermal treatment water and organic material evaporates from the drill cuttings. The organic material may decompose if the temperature is too high. Decomposed organic material cannot be reused. Steam treatment is a more gentle thermal treatment that avoid decomposition of organic material including oils. Current statutory requirements require that the residual proportion of oil must be less than 10 g/kg of dry substance for allowing the cuttings to be disposed into the surroundings.
It is obvious, particularly when offshore drilling operations are involved, that transport and subsequent treatment of the drill cuttings are costly and environmentally dubious, insofar as transport and at least some of the known cleaning operations require significant amounts of energy.
It is known to heat drill cuttings directly and mechanically with a hammermill. The mechanical energy provided by the rotating hammers, heat the material. It is also known to heat drill cuttings indirectly in a rotary kiln where the material is exposed to a hot surface.
It is known to remedy polluted soils by heat treatment. This is commonly done in thermal desorption units comprising a heated flow through chamber with a conveying screw. The chamber wall may be heated or the conveying screw may be heated or both. The contaminated material is thereby exposed to at least one hot surface. Drill cuttings may also be cleaned by this method.
Many of these cleaning techniques rely on steam distillation. Water is added to the material if the material to be cleaned is too dry. As an example, experience has shown that the use of a hammermill will not work with dry material. Addition of water improves the process due to that the water contributes to the steam distillation. However, addition of water contributes to a less efficient process due to the heat capacity of water and to the relative high enthalpy of evaporation of water.
In addition to the effect of steam distillation, some compounds are also separated by entrainment. Entrainment is the effect of a fluid boiling inside the pores of the material creating small droplets of fluid in addition to the steam (shock poaching). The steam and small droplets are separated from the substrate.
It is known to use microwave ovens for heating of food items. Consumer microwave ovens usually operates at 2.45 GHz (12.2 cm wave length). Industrial and commercial microwave ovens operate at 915 MHz (32.8 cm wave length). Molecules that are electric dipoles, such as water, absorb electromagnetic energy from the microwaves. Such molecules rotate as they try to align themselves with the alternating electric field of the microwaves. A susceptor is a material that absorbs electromagnetic energy and convert the energy to heat.
Heating oil contaminated drill cuttings with microwave radiation has been performed for a numbers of years by different institutions. The University of Nottingham successfully built a drill cuttings treatment plant that utilizes a 100 kW magnetron supplying microwaves at 896 MHz. Nitrogen gas is used as sweep gas in order to increase the entrainment process. The pilot treatment plant manage to treat around 800 kg drill cuttings per hour while reducing the retained oil on cuttings (ROC) to below one percent. (Pereira IS. 2013. Microwave processing of oil contaminated drill cuttings. PhD thesis, University of Nottingham, UK). A commercial plant is provided by the company Rotawave Ltd. Shang et al. disclose use of microwaves for the treatment of oil-contaminated drill cuttings. Water/salt water was heated by exposure to microwaves and the rapid heating of the water provided the sensible and latent heats for vaporisation of the oil-based materials. The authors conclude that the concentration of the water within the samples appears to limit the possible maximum oil removal. Increased moisture content within the samples resulted in improved oil removal. The authors do not suggest other additives than water/salt water as susceptors. (Shang, H. et al. 2006. Microwave treatment of oil-contaminated North Sea drill cuttings in a high power multimode cavity. Separation and Purification Technology, 49:84-90.)
It is known to use heating by infrared radiation in addition to heating with microwaves. The purpose of the IR radiation is to further heat and separate oil from drill cuttings when all the water has evaporated. When all water on drill cuttings has been removed, microwave radiation has no heating effect in principle. This is valid if the oil or drill cuttings has no dipole moment, i.e. the material is transparent to microwaves. The IR radiation, which is in principle heat radiation at a certain frequency, would allow for heat generation and thus further evaporate oil from the drill cuttings. The combination of microwave radiation and IR radiation is a technology that has previously been utilized by other industries (Kowalski S. J., Mierzwa D. 2009. Convective drying in combination with microwave and IR drying for biological materials. Drying Technology, 27:1292-1301).
The principle of steam distillation applies for two immiscible fluids (e.g. water and oil) in accordance with Dalton's law. As a result, each fluid will start boiling at a reduced temperature and the vapour pressure of the two fluids will be shared.
Antoine's equation describes the relation between vapour pressure and temperature for pure compounds:
            log      10        ⁢    p    =      A    -          B              C        +        T            P is the vapour pressure, T is temperature (° C.) and A, B, and C are component specific constants. Values for a particular component may be found in C. Yaws. 2007. “The Yaws Handbook of Vapor Pressure: Antoine Coefficients”, 1st Edition. Gulf Publishing Company.
Dalton's law represents the law of partial pressure. It states that in a mixture of non-reactive gases, the total pressure exerted is equal to the sum of partial pressures of the individual gases: Ptotal=Σi−1nP1 or Ptotal=P1+ . . . +Pn where p1, p2, . . . , pn represent the partial pressure of each component.
The ideal gas law: PV=nRT, where P is the pressure of the gas, V is the volume of the gas, n is the amount of substance of the gas (number of moles), R is the universal gas constant, T is the temperature.
For a mixture of two gases:
                                                        P              pol                        ⁢                          V              pol                                                          P              org                        ⁢                          org                                      =                                            n              pol                        ⁢                          RT              pol                                                          n              org                        ⁢                          RT              org                                                          (                  Eq          .                                          ⁢          1                )            where “pol” represents any polar compound and “org” represents any non-polar compound in vapour phase.
The gases occupy the same volume Vpolw=Vorg. The temperature is also the same as the two compounds boil at the same temperature. Eq. 1 than simplifies to:
                                          P            pol                                P            org                          =                                            n              pol                                      n              org                                .                                    (                  Eq          .                                          ⁢          2                )            
The major advantage of having a high temperature is the increased partial pressure contribution from the oil on the cuttings. This will in practical terms mean that significantly less polar liquid is required to evaporate the oil from the cuttings.
It is well known in the art that fluids have different specific heat capacities and different enthalpy of vaporization. Examples are shown in Table 1.
TABLE 1Comparison of different compounds with an electric dipoleSpecific heatBoilingEnthalpy ofcapacitypointvaporizationCompound(J/kg · K)(° C.)*(kJ/kg)Water4181.31002257Ethylene glycol (MEG)2408.6197.3924Diethylene glycol (DEG)2306245.8628Triethylene glycol (TEG)2198285481Citric acid1179310Glycerol2409.6290996Propylene glycol (MPG)2510188.2880Dipropylene glycol (DPG)2180230.5338Tripropylene glycol (TPG)1970265.1184*At 1 atmospheric pressure